The morning news. [volume] (Savannah, Ga.) 1887-1900, 1887-08-23
TESTING A GREAT LENS.
A VISIT TO THE WORKSHOP OF
THE LATE ALVAN CLARK.
Polishing Glass With the Palm-The
Rough Machinery He Used to Test
Telescopes - Small Telescopes OF
Greater Value.

In 1882 called to see Alvan Clark, the
distinguished constructor of telescopes, who
has just died. found him at his home in
Cambridgeport, Mass His sons were busy
putting the finishing touches to the great
telescope object glass then being made for
the Russian Astronomical Observatory at
Pulkova. The Pullxova objective was to be
placed in temporary mounting that very
evening to be tested for the first time on
celestial objects.
found Mr.. Clark genial Intelligent old
man, who had relinqushed all manual la
Dor in the business, but was still hale and
hearty, and personally directed the work.
He told me Of his visits to England, and his
interviews with Sir John Herschel and with
Lord Ross, who built the great reflector in
ireland. In an upper story found the
Rochester telescope, made by Prof. Lewis
Swift, the cost of which was paid by Mr..
Warner of the same city. This objective
was sixteen inches in diameter. One of
the sons showed the workshops and ex-
planned their methods The operation of
grinding these object glasses was carried on
in the basement, and it appeared to be a
very simple matter in its earlier stages. An
assistant was rubbing with both hands
disc of glass about eight inches in diameter
upon prepared surface covered with red
polishing material The work seemed very
monotonous. Mr. Clark said that any one of
common intelligence could grind a lens, and
he showed me one that had been sent to him
by an amateur. Mr. Alvan G. Clark said
it was a very creditable performance.
1t is in the after correction and perfecting
of the objective which calls for the special
skill which has made Alvan Clark's name
so famous The large objectives, such as
the Pulkova and Lick glasses, are ground
II,
work in opposite directions, and are so ar
ranged that any part of the glass surface
may be reached, doing the work. The final
polish which is of the most delicate nature
imaginable, is done with the surface of the
hand. Mr.. Clark went so far as to say that
one revolution of the hand in excess would
change the correction.
On table was the finished Pulkova lens,
which weighed $50 pounds, and consisted of
two lenses, each 80 inches in diameter. Cen
orally these lenses are made to accurately
fit, and are joined together with Canada
balsam, but in such large glasses as the Pull
kova and the Lick they are fitted in a
metal frame with an adjustment so that
they can be made to approach each other,
or otherwise. If single lens were used,
the object inspected would be fringed with
various artificial colors and other defects
due to spherical alteration, but by the
simple artifice of using two kinds of glass
these defects are cured to a certain extent,
and nearly perfect image is secured.
When the evening was sufficiently adj
vanced the great Pulkova glass was placed
in its temporary fitting in the garden. There
was no moon and the darkness was intense.
The glass was brought out on a four wheel
hand truck and lifted into the tube by five
men and fixed by revolving it in the screw
fitting The tube was 45 feet long and
weighed with the attending fittings about
seven tons. Two piles of brickwork sup
ported the whole. There was no clockworl1
movement and the toughest apparatus was
employed, the telescope was raised and
moved by a guide rope, the motion of an
| equatorial movement being initiated by
using q common windlass As the motion
of the earth caused the object to pass across
| the field of the telescope, the observer gave
the order "follow," when a slight turn of
the windlass kept the object in view. Such
were the rough appliancesused to test this
100000 lens.
The planets had all set, and had to be
satisfied with view of fixed star, which
is an excellent object for testing the opti-
cal properties of a lens, but verv uninter
esting otherwise, as the largest telescope can
make little impression on fixed star; no
disk Gun be seen, merely Speck of light.
The star selected was small one, and barely
visible as a pale, minute object On look
| ing at it with this magnificent instrument
its wonderful light-gathering powers were
at once evident, for the star shone with the
| lustre and brilliancy of an electric light. 1t
was an object which brought out all the inn
| projections of the glass, and to the eyes of
| Mr., Clark and his sons many were evident,
and, it was said, two months work was
necessary to correct them During the trial
the lens was lowered and five men revolved
| the glass in its fitting. On its being placed
,,~eAl h. ,k.~.,4
| to make another test when the old man
shouted: "Wait boys, let her cool"
| was curious to know what this could
| mean, and Alvan Clark explained that the
| correction was so delicate that the heat
from the hands of the five men holding the
metal case of the objective would change
the correction, so it had to cool.
On the front or crown glass lens was a
very marked flaw, due to the tapping over
of strata of glass in cooling. Bymeasure-
ment I found it to he Il inches long by Is
of an inch in diameter, almost in the centre
of the lens, near the front surface. This
was very provoking circumstance in a
100000 glass Mr. Clark had ordered an
other disc of glass, but Prof Won Otto
Streuve decided to accept the damaged one
at least for time, as it seemed very doubt
full when another glass might he expected
to be made in France. This flaw would not
alter the definition of the lens, but would
merely stop fractional part of light pass
ing through the lens.
In a conversation with Mr.. Clark on the
advantages of immense telescopes, such a.
this one, he admitted that the Telescopes OF
moderate aperture, say from 10 to - inches
were preferable for general use, even foI
the highest purposes. He spoke with pride
OF such an instrument he had made, with
which he had seen all that could be seer
even with colossal teleScScopes. Only one
discovery has been made with the Washing
ton telescope of 20 inches diameter DURING
its many years of use. The Russian Pul
kova instrument, delivered by the Clark.
in 1882, has never been heard from, and
those who expect immediate and extraordi
nary results from the Lick 36-inch objective
will probably be disappointed These huge
telescopes are great light gatherers, apl
useful for Photographic and spectroScopie
work, but their definition of the surface OF
the moon and the planets is Disappointing
A story is told of French astronomer
who Travelled hundreds of miles to look
through the great Ross telescope. When
asked what he would like TO see, he called
for the planet Saturn, which was then IN
good position. On looking through the tele.
scope finding the wretched definition of the
object, he said
"You tell me have looked at the planet
Saturn, so | must believe | have done so
but was not aware of the fact"
JOHN MIcHELs.
ut..s,